This invention relates to method and means for compensating for track eccentricity in a rotating recording member during playback. It relates more particularly to a technique for compensating for both tracking and speed errors due to such eccentricity so that more information can be packed on the recording member.
In many recording systems, the recording and information storage are done in a disk-type format. For example, stiff and floppy disks are used for digital storage, both magnetic and optical disks are used to store video information and acoustical information is stored mechanically on a phonograph disk. In many cases, it is necessary to position a playback transducer or read head at a desired data track on the recording member with great precision. This is because the information tracks on such recording members are often in the form of concentric circles or spiral convolutions that may be only on the order of five milli-inches wide and spaced an even lesser distance apart on the member. Also in some instances, the recording track contains precisely spaced apart timing or index signals used for control purposes. Obviously, then, the minimum track and index signal spacings are directly dependent upon the accuracy with which the transducer can be positioned relative to the recording member.
When information is being retrieved from disk-type recording members there may be a lack of concentricity between the rotating disk and the circular or spiral recording tracks previously recorded on that disk. In other words, the center of the tracks may be offset from the center of rotation of the disk during playback. The problem arises because the information may be recorded concentrically on the disk on a given machine. Then for one reason or another, the disk may be removed from that machine prior to playback. When it is time to retrieve information from that disk, it is sometimes quite difficult to return the disk to its true or original center of rotation when it is placed in the same or a different machine. Consequently the center of the information tracks during playback no longer corresponds to the center of rotation of the disk. As a result, the recording tracks "wander" radially relative to the playback transducer which is normally maintained at a fixed distance from the center of rotation of the disk. Accordingly the output signal from the transducer corresponding to the information on the selected track is amplitude modulated at twice the frequency of angular rotation of the disk and in extreme cases, an adjacent track may move opposite the transducer, in which case the output from the transducer would be completely erroneous.
In addition to the radial tracking error just described, track eccentricity during playback gives rise to a track speed or time error. In other words, as the track wanders radially inward and outward with respect to the transducer as the disk rotates at a constant angular velocity, the speed of the track past the transducer decreases and increases correspondingly, i.e. the track goes too slow half the time and too fast half the time. Accordingly, information read from the track, e.g. control or index points, varies time-wise relative to a fixed time reference such as timing signals synchronized to the rotation of the disk. Accordingly the index information retrived from the disk would be unsuitable for controlling downstream equipment. Also, if the data on the disk is frequency modulated, during playback from an eccentric track the retrieved signals are frequency modulated at a frequency equal to that of the rotation of the disk giving rise to other filtering or control problems.
Finally the lack of concentricity between the centers of the recording track and the rotation of the recording member during playback causes the azimuth angle between the head and recording member to vary over each revolution of the disk member. This may result in degradation of the output signal from the playback transducer and excessive parts wear.
Several types of electrical systems have been proposed to compensate for tracking error during playback of a rotating recording member such as a disk. Examples of such are shown in the following U.S. Pat. Nos.: 3,246,307; 3,840,893; 3,854,015 and 3,864,740. One type of system utilizes a playback transducer having two or more sensing coils aligned perpendicular to the recording track on a disk. The outputs of the coils are then compared to develop a tracking error signal. The magnitude and polarity of this signal varies as the track wanders radially inward or outward from its centered position opposite the transducer. The error signal is then used to control a positioning device which moves the playback transducer radially inward or outward as needed to maintain the track in its centered position opposite the transducer.
Other such systems employ a pair of transducers arranged perpendicularly to a disk recording track carrying a prerecorded frequency. Each transducer produces a signal at the track frequency whose level is proportional to the portion of the transducer opposite that track. The two signal levels are then compared to produce a difference signal that controls an actuator to move the transducers radially inward or outward to maintain them centered over the track.
Still other systems use optical detecting means and the same sort of servo loop to compensate for track eccentricity.
None of these conventional systems provides an exact compensation for track eccentricity. In the first place, being servo-following devices, they do not anticipate or measure the amount of the eccentricity in advance to set up an exact compensation for the tracking error. Furthermore, they do not compensate at all for the track speed or timing variation due to track eccentricity. Consequently the information retrieved from the disk may still vary in time and frequency so as to produce errors in or malfunction of equipment controlled by the information on the disk.
Still further, these prior systems do not provide any correction at all for the azimuth variation between the transducer and disk due to track eccentricity.